The present invention relates to novel water-dispersible graft copolymers which have a main chain which forms a stabilizing layer and side chains which constitute a dispersed layer and are useful as paint-film-forming components, to a method for making them, and to water-based paint which includes an afore-mentioned water-dispersible acrylic graft copolymer.
Water-based paints have come to be widely adopted instead of solvent-based paints on account of their superiority from the point of view of environmental protection and worker safety. For example, in the field of metallic paint painted onto the outside panels of cars, etc., solvent-based paints using large quantities of organic solvents are being progressively replaced by water-based paints in which water is used as the solvent.
The hydrophillic resins generally employed in water-based paints or water-based paint compositions are random copolymers having a quantity of carboxyl groups necessary to render them hydrophillic; and dispersions formed by neutralizing such a polymer with a basic substance and dispersing it in an aqueous medium are used in water-based paint.
However, although water-based paints from such aqueous dispersions, such as water-based paint from an aqueous dispersion of an acrylic type random copolymer for example, are suitable for improving appearance because pigment dispersion and the Theological properties of the paint liquid are good and they form paint films of outstanding gloss and smoothness, the durability of the paint film is low because of the high acid number of the acrylic type polymers. When the viscosity of water-based paints obtained from these is adjusted to make it suitable for painting, the solids concentration thereof decreases dramatically, and painting faults such as drip and uneven application, etc., are prone to be produced.
In order to improve such points, water-based paint compositions have been offered which include an acrylic graft copolymer from a hydrophillic component and a hydrophobic component. For example, in Japanese Unexamined Patent 56-49760 water-based paint compositions are described which contain as the resin component an acrylic graft copolymer obtained by copolymerizing an acrylic polymer of acid number 40-200 containing unsaturated groups, obtained by reacting an unsaturated monomer containing a glycidyl group with a carboxy-functional acrylic polymer, and a monomer combination combining an .alpha.,.beta.-ethylenic unsaturated carboxylic acid and another copolymerizable unsaturated monomer to give an acid number of .ltoreq.30, such that the difference in acid number in the reaction product composition is 25-200, and by this means making a graft copolymer which has an acid number of 15-40 and a glass transition temperature of -10.degree. C. to 70.degree. C.
However, in making an acrylic graft copolymer such as described above it is necessary first to react a carboxy-functional acrylic polymer with an unsaturated monomer containing a glycidyl group, and then to graft copolymerize a monomer combination; therefore the manufacturing process is complicated by a 2-stage reaction process. Moreover, in the preliminary reaction of an acrylic polymer with an unsaturated monomer containing a glycidyl group, it is necessarily to employ a polymerization stopper so that the grafted chains are not extended by free radical polymerization of the unsaturated monomer, and in the subsequent reaction it is necessary to add a large quantity of polymerization initiator so that free radical polymerization will proceed to form a grafted chain even though the polymerization stopper may be present; and polymerization side reactions become prone to occur. When an acrylic graft copolymer made by the method described above is used as the main resin in a water-based paint the paint film formed may yellow due to contamination of the paint with the polymerization stopper. Improvement in ease of painting would also be desirable.